tryin to find the right frequency

Then it's not officially tuned.

If you are trying to use the truck's resonant frequency to augment the SPL of the subs, then you are working on a competition SPL vehicle.  Your best bet is to build a precisely tuned bandpass for that purpose.

But if you want to use the truck for listening purposes other than competition, and I'm betting you do, then finding the resonance of the truck is not what you should be after.  It would likely be too high for where you would want the subs to be tuned at.  For normal (albeit LOUD) listening, and with a vehicle with that much space and using that much of it for subwoofers,  you should be working on an enclosure that will fully complement those subs and provide a tuning frequency near 20 Hz.

Wait.........

I just looked into WinISD Pro to see if L7's were listed and yes, they have the lot of them.  Looks like the L7 wants a very large sealed box.  If the enclosure volume is too small for each of those subs, the frequency (Fsc for sealed) goes up....so where you need huge volumes of airspace to try to smooth the frequency response plot, a more normal amount of airspace causes a bandpass effect with this sub, with a very definite peak in response.  You don't need to build a bandpass, you get it anyway with this sub in a sealed enclosure.

Trying to vent these subs looks like a nightmare, and even with huge ports the peak in response looks like it will be higher than with sealed.

If this is a comp vehicle you might need professional hands-on help.  If it's not, what you don't need are 4 of these L7 15's.  By using a semi-sealed environment (a wall), you are not getting the sound you should....and if you enable the volume of airspace to each sub that they need to sound more well-rounded, you will find that even in a Tahoe your space is limited.

If you're peaking at about 50 Hz now, that sounds about right for what you would typically get with these subs.  I don't know how much RMS power these subs have available to them, but if you're at 1500 watts or less you might benefit by dropping two of them from the lineup and working on massive, well-braced enclosures for the remaining two.   It depends on your purpose of having them...

What are they, dual 2 ohm or 4 ohm?